COS 318: Operating Systems File Systems: Abstractions and Protection Jaswinder Pal Singh Computer Science Department Princeton University (http://www.cs.princeton.edu/courses/cos318/)
Topics What s behind the file system: Storage hierarchy File system abstraction File system protection 3
Traditional Data Center Storage Hierarchy Clients LAN Server SAN Storage WAN Remote mirror Onsite Backup Storage Offsite backup 4
Evolved Data Center Storage Hierarchy Clients LAN Network Attached Storage (NAS) w/ snapshots to protect data WAN Remote mirror Onsite Backup Storage Offsite backup 5
Alternative with no Tape Clients LAN Network Attached Storage (NAS) w/ snapshots to protect data WAN Remote mirror Onsite Backup Deduplication Capacity and bandwidth optimization WAN Remote Backup 6
Public Cloud Storage Hierarchy WAN WAN Interfaces Clients Geo-plex Examples: Google GFS, Spanner, Apple icloud, Amazon S3, Dropbox, Mozy, etc 7
Topics What s behind the file system: Storage hierarchy File system abstraction File system protection 3
Revisit File System Abstractions Network file system! Map to local file systems! Exposes file system API! NFS, CIFS, etc Local file system! Implement file system abstraction on block storage! Exposes file system API Volume manager! Logical volumes of block storage! Map to physical storage! RAID and reconstruction! Exposes block API Physical storage! Previous lectures Network File System Local File System Volume Manager Physical storage 8
Volume Manager Group multiple storage partitions into a logical volume! Grow or shrink without affecting existing data! Virtualization of capacity and performance Reliable block storage! Include RAID, tolerating device failures! Provide error detection at block level Remote abstraction! Block storage in the cloud! Remote volumes for disaster recovery! Remote mirrors can be split or merged for backups How to implement?! OS kernel: Windows, OSX, Linux, etc.! Storage subsystem: EMC, Hitachi, HP, IBM, NetApp 9
File versus Block Abstractions File abstraction Byte oriented Named files Users protected from each other Robust to machine failures Disk/Volume abstraction Block oriented Block numbers No protection among users of the system Data might be corrupted if machine crashes Emulate block storage interface Support file systems, database systems, etc. 10
File Abstraction: File Structures Byte sequence! Read or write N bytes! Unstructured or linear Record sequence! Fixed or variable length! Read or write a number of records Tree! Records with keys! Read, insert, delete a record (typically using B-tree) 11
File Abstraction: File Types ASCII Binary data! Record! Tree! An Unix executable file header: magic number, sizes, entry point, flags text data relocation bits symbol table Devices Everything else in the system 12
File Abstraction: File Operations Operations for sequence of bytes files! Create: create a file (mapping from a name to a file)! Delete: delete a file! Open: authentication! Close: done with accessing a file! Seek: jump to a particular location in a file! Read: read some bytes from a file! Write: write some bytes to a file! A few more operations on directories: later Implementation challenges! Keep disk accesses low! Keep space overhead low 13
File Access Patterns Sequential (the common pattern)! File data processed sequentially! Example: Editor writes out a file Random access! Access a block in file directly! Example: Read a message in an inbox file Keyed access! Search for a record with particular values! Usually not provided by today s file systems! Examples: Database search and indexing 14
File System vs. Virtual Memory Similarity! Location transparency! Size "obliviousness"! Protection File system is easier than VM in some ways! File system mappings can be slow! Files are dense and mostly sequential, while page tables deal with sparse address spaces and random accesses File system is more difficult than VM in some ways! Each layer of translation causes potential I/Os! Memory space for caching is never enough! File size range vary: many < 10k, some > GB! Implementation must be reliable 16
VM Page Table vs. File System Metadata Page table Manage the mappings of an address space Map virtual page # to physical page # Check access permission and illegal addressing TLB does it all in one cycle File metadata Manage the mappings of files Map byte offset to disk block address Check access permission and illegal addressing Implemented in software, may cause I/Os 15
Topics What s behind the file system: Storage hierarchy File system abstraction File system protection 3
Protection: Policy vs. Mechanism Policy is about what Mechanism is about how A protection system is the mechanism to enforce a security policy! Same set of choices, no matter what policies A security policy defines acceptable and unacceptable behaviors. Examples: A given user can only allocate 4GB of disk storage No one but root can write to the password file A user is not allowed to read others mail files 17
Protection Mechanisms Authentication! Identity check Unix: password Credit card: last 4 digits of credit card # + SSN + zipcode Airport: driver s license or passport Authorization! Determine if x is allowed to do y! Need a simple database Access enforcement! Enforce authorization decision! Must make sure there are no loopholes 18
Authentication Usually done with passwords! Relatively weak, because you must remember them Passwords are stored in an encrypted form! Use a secure hash (one way only) Issues! Passwords should be obscure, to prevent dictionary attacks! Each user has many passwords Alternatives? 19
Protection Domain Once identity known, provides rules! E.g. what is Bob allowed to do? Protection matrix: domains vs. resources File A Printer B File C Domain 1 R W RW Domain 2 RW W Domain 3 R RW 20
By Columns: Access Control Lists (ACLs) Each object has a list of <user, privilege> pairs ACL is simple, implemented in most systems! Owner, group, world Implementation considerations! Stores ACLs in each file! Use login authentication to identify! Kernel implements ACLs Any issues? 21
By Rows: Capabilities For each user, there is a capability list! A lists of <object, privilege> pairs Capabilities provide both naming and protection! Can only see an object if you have a capability Implementation considerations! Architecture support! Capabilities stored in the kernel! Capabilities stored in the user space in encrypted format Issues? 22
Access Enforcement Use a trusted party to! Enforce access controls! Protect authorization information Kernel is the trusted party! This part of the system can do anything it wants! If there is a bug, the entire system could be destroyed! Want it to be as small & simple as possible Security is only as strong as the weakest link in the protection system 23
Some Easy Attacks Abuse of valid privilege! On Unix, super-user can do anything Read your mail, send mail in your name, etc.! If you delete the code for COS318 project 5, your partner is not happy Spoiler/Denial of service (DoS)! Use up all resources and make system crash! Run shell script to: while(1) { mkdir foo; cd foo; } Listener! Passively watch network traffic 24
No Perfect Protection System Cannot prevent bad things, can only make it difficult to do them There are always ways to defeat protection! burglary, bribery, blackmail, bludgeoning, etc. Every system has holes 25
Summary Storage hierarchy can be complex! Reliability, security, performance and cost! Many things are hidden Key storage layers above hardware! Volume or block storage! Local file system! Network file system Protection! ACL is the default in file systems! More protection is needed in the cloud 26